Patent GB-711956-A (GB)
Title: Improvements relating to processes of and apparatus for spinning polyamides
IPC: D01F6/60
Filed: 1950-08-21
Abstract: <PICT:0711956/IV (a)/1> <PICT:0711956/IV (a)/2> <PICT:0711956/IV (a)/3> Synthetic linear polyamides derived from o -aminocarboxylic acids, or from derivatives thereof from which such polyamides can be formed, or from their lactams, are melt-spun by allowing the liquid polymer (which has not been allowed to solidify after the polymerization of the monomer) to flow by gravity in a thin layer or layers down one or more vertical or inclined or partly vertical and partly inclined surfaces while being heated, exposing the melt during its passage over the surface either to a current of inert gas or to a high vacuum, whereby the low molecular weight fractions are evaporated, and allowing the molten polymer thereafter to accumulate in a small pool, the surface of which is also exposed to high vacuum or covered by an inert gas, the polymer being conducted from the pool to the spinning nozzle.  As shown in Fig. 2, the polyamide melt is delivered by the pump 1 and nozzle 16 to the heated surface of the thinlayer evaporator 2. Alternative forms for the heated evaporation surfaces are single or double (base to base) cones or a sphere.  An inert gas is introduced into the evaporator at 4 and is withdrawn from the evaporator at 5 carrying with it the low molecular weight fractions.  The residual melt 17 is collected in the pool 6 which is formed by the upper part of the pump block 12.  By means of a spinning pump 3 the melt 17 is forced through the filter 13 in the spinning nozzle 14 to form the spun filaments 15.  The evaporator is surrounded by an insulated heating jacket 11.  The amount of melt present in the pool 6 preferably does not exceed that required for half an hour's spinning, in order to minimize the re-formation of monomers in the melt.  In the modification shown in Fig. 4, the melt is distributed by the pump 1 and two branch pipes to two concentric annular slits 20 from which it spreads over the inner and outer surfaces of the vertical concentric cylinders 21.  In the modification shown in Fig. 3, the melt is discharged by the pump 1 on to a horizontal circular disc 18, which is rotated by a driving mechanism 19 at a sufficiently high speed to throw the melt off by centrifugal force and thus distribute it uniformly over an annular area at the upper end of the inner surface of the heated vertical cylinder 11.  From this zone the melt flows down in a layer of uniform thickness and collects in the pool 6.  The evaporator may be heated electrically, e.g. by induction.  Inert gas passed through the evaporator is preferably pre-heated and may be at atmospheric pressure or somewhat above.  The inert gas, preferably nitrogen, is re-circulated through the apparatus after the separation therefrom of the volatile fractions.  A second surface may be disposed at a distance of a few millimetres from and concentric with the evaporting surface, such as that shown at 2 in Fig. 2. This second surface functions as a condensing surface and is maintained at a temperature lower than that of the evaporator surface but above the melting point of the low molecular weight fractions, so that the latter may run down and off the surface. The melt is not fed to the second surface, between which and the pool 6 there is provided an annular channel in which the low molecular weight fractions collect and from which they are conveyed by means of a pipe.  The second or condensation surface is particularly effective when the evaporation is carried out at pressures of 0.1 to 0.001 mm. Hg.  The polyamide melt may be prepared in a number of autoclaves connected in parallel with a collecting vessel, e.g. 6 autoclaves each discharged into a collecting vessel every 4 hours.  The temperature of the evaporator is preferably 30 DEG C. higher than the melting point of the polyamide to be spun, but not above 280 DEG C.  In an example polymerized e -caprolactam which still contains 12-15 per cent. of low molecular weight material is heated to 240 DEG C. and distributed to 12 spinning heads provided with thin-layer evaporators of the type shown in Fig. 2, each evaporator having a superficial area of 1500 sq. cm. and being heated to 250 DEG C.  The delivery pump and spinning pump are adjusted to deliver 11 grams of melt per min. Nitrogen preheated to 250 DEG C. is passed through the apparatus at a pressure of 50 cm. water column and a rate of 3 litres per minute. The spun filaments contain 3-4 per cent. of low molecular weight material.  By the process described, low-titre polyamide filaments may be spun directly from the melt in the polymerization vessel without intermediate solidification of the material and solvent extraction of low molecular weight fractions therefrom.